void setup() {
Serial.begin(9600);
// get data from Serial.readStringUntil('\n'); to memory
String data="FFFFFFFF 00FFFFFF 0000FFFF 000000FF";
// store length of data
int iend = data.length() - 1;
// current begin index
int index=0;
// next index
int next_index;
do{
next_index=data.indexOf(' ',index);
String hexcode=data.substring(index, next_index);
index=next_index+1;
unsigned long bigNumber = (unsigned long)strtoul(hexcode.c_str(), 0, 16);
// add number to array
Serial.println(bigNumber);
}while((next_index!=-1)&&(next_index<iend));
}
void loop() {
}